I think (as with other things!) that Duke/ManxRadio/ACUE have got it wrong: if the excellent commentaries were freely available on the net, and appropriate publicity given to that fact, a very wide audience all over the world could listen in. This would not only maintain/increase interest in the TT as a major sporting event, but provide a great platform to sell advertising on the station - not just for 'Mylchreest's Fish & Chip Shop, Valley Parade' and so on, but world-wide major brands, like Coke and Kellogs.
You could compare it to football (Yuk!), where sure there are some fanatics who will subscribe to Setanta or MUFCTV or whatever, but in terms of massive audiences for big games, these are found watching ITV, and those annoying ads that are all round the grounds as well as in the intervals.
Making it pay-per-listen keeps it as a small niche operation, and does nothing to build the event.